In a conventional magnet-type rodless cylinder as is well known, inner magnets are disposed on a piston accommodated in a tube, and outer magnets or magnetic members are provided in a slide body disposed on the outer side of the tube. The piston and the slide body are coupled together by the magnetic coupling force between the inner magnets and the outer magnets or the magnetic members. As the piston is moved in the tube by a fluid supplied to the tube, such as compressed air, the slide body on the outer side of the tube moves following the piston.
In conventional magnet-type rodless cylinders, in general, the slide body moves based on the movement of the piston (i.e., inner magnets), the slide body being attracted by the moving inner magnets. Here, the magnitude of the attracting force is an index that represents the conveying ability of the magnet type rodless cylinder and is usually called a “magnetic holding force”.
FIG. 9 is a sectional view schematically illustrating the structure of a conventional general magnet type rodless cylinder.
Referring to FIG. 9, four outer magnets 102 are arranged in a slide body 101 on the outer side of a tube 100, and four inner magnets 104 are arranged in a piston 103 on the inner side of the tube 100, respectively, holding yokes 105 among them in the axial direction. The four outer magnets 102 and the four inner magnets 104 are so arranged that the same poles oppose each other in the axial direction, the inner magnets 104 and the outer magnets 102 being opposed to each other at their different poles.
Here, the “magnetic holding force” is defined as the force in the axial direction acting on the slide body 101, when the inner magnets 104 are displaced in the axial direction of the tube, relative to the slide body 101 (outer magnets 102) by exerting fluid pressure on the piston 103 while the slide body 101 is fixed, so that the slide body does not move in the axial direction.
In a static state where no fluid pressure is applied as shown in FIG. 4B, i.e., in a state where the four inner magnets 104 and the four outer magnets 102 are at positions, which are in alignment with each other in the radial direction without being deviated in the axial direction, the magnetic holding force becomes zero at point A. The magnetic holding force increases with an increase in the deviation between the inner magnets 104 and the outer magnets 102 in the axial direction, and becomes maximum Max (point B) when the deviation is about one-half the pitch L of the arrangement of the magnets 102, 104 in the axial direction.
According to another conventional magnet-type rodless cylinder as shown in FIG. 10, outer magnets are omitted by using a magnetic material for forming the slide body 101 and protuberances 101a which oppose the yokes 105 is provided for the slide body 101. In the magnet type rodless cylinder of this type, magnetic holding force is zero in a state where no fluid pressure is exerted.
Japanese Registered Utility Model No. 2514499 discloses a magnet type rodless cylinder in which a plurality of cylinder tubes are arranged in parallel, pistons are disposed in the cylinder holes in the cylinder tubes, a slider is arranged so that it strides all tubes, and the plurality of pistons are magnetically coupled to the slider.
However, in the above conventional magnet-type rodless cylinders, the inner magnets 104 and the outer magnets 102 in a static state are at rest where the magnets attract each other in the radial direction and are in alignment. That is, no displacement (deviation) occurs in the axial direction between the inner magnets 104 and the outer magnets 102, and magnetic holding force is zero as described with reference to FIG. 4B.
Therefore, if the piston 103 is attempted to be moved in this state, no driving force is exerted on the outer magnets 102 until a “deviation” occurs in the axial direction. Therefore, conventional magnet-type rodless cylinders lack smoothness in the movement due to a stick-slip phenomenon at the start of the slide body 101. The above problem also occurs in the rodless cylinder of the structure omitting the outer magnets as shown in FIG. 10.
In the magnet-type rodless cylinder of the above Japanese Registered Utility Model No. 2514499, further, the plurality of cylindrical tubes are arranged maintaining a considerable distance therebetween, and the inner magnets of the pistons accommodated in the respective cylindrical tubes do not exert magnetic force on each other. Accordingly, the inner magnets of the pistons are in alignment with the outer magnets of the slide body completely facing them in the radial direction, and are presumably not deviated in the axial direction. Therefore, the above stick-slip problem occurs.